1. Field of the Invention
The present invention relates to an electric power steering apparatus so configured as to transmit a driving force produced by an electric motor to a steering mechanism as a steering assist force.
2. Description of Related Art
Electric power steering apparatuses so configured as to perform steering assist by mechanically transmitting a driving force produced by an electric motor to a steering mechanism by a gear mechanism (a speed reduction mechanism) or a direct drive system have been conventionally employed.
In such electric power steering apparatuses, assist characteristics for determining the relationship between a steering torque applied to a steering wheel and an assist torque target value given to the steering mechanism from the electric motor have been previously determined, and are stored in a memory as an assist map. The assist torque target value corresponding to the steering torque is read out of the assist map, and the electric motor is driven and controlled on the basis of the read assist torque target value.
The assist characteristics are determined such that the larger the steering torque is, the larger the assist torque target value becomes, as shown in FIG. 7. A positive value is assigned to the steering torque with respect to a rightward steering direction, while a negative value is assigned to the steering torque with respect to a leftward steering direction, for example. The assist characteristics are so determined that a positive value of the assist torque target value corresponds to the steering torque taking the positive value, and a negative value of the assist torque target value corresponds to the steering torque taking the negative value.
When the assist torque target value is a positive value, such a steering assist force as to steer a steerable wheel rightward is exerted on the steering mechanism. Conversely, when the assist torque target value is a negative value, such a steering assist force as to steer a steerable wheel leftward is exerted on the steering mechanism. When the steering torque takes a value in a dead zone in the vicinity of zero, the assist torque target value is made zero.
In such electric power steering apparatuses to which such assist characteristics are applied, when return stroke steering for rotating a steering wheel toward a steering angle midpoint is done, such a steering feeling (a so-called spring feeling) that the steering wheel is returned to the steering angle midpoint strongly than a driver intends is produced. That is, when return stroke steering is done, a steering torque is reduced and correspondingly, an assist force is reduced. Accordingly, the steering wheel is returned to the steering angle midpoint strongly by an inverted input from the steerable wheel.
This problem can be solved by increasing the slope of an assist characteristics curve such that a larger assist torque target value is set with respect to the steering torque. In this case, however, a responsive feeling at the time of forward stroke steering is degraded.